1. Field of the Invention
This invention relates to a novel apparatus for the development of resist patterns on substrates, for example lithographic printing plates, and to methods of using such novel apparatus. The invention is of particular, but not exclusive, interest in relation to the development of lithographic printing plates, especially thermal lithographic printing plates.
2. Background Information
The objects to be developed using the apparatus of this invention are commonly printing plates, but this is not a necessity. They could comprise any substrate which requires the development of a resist pattern in order to produce a required pattern in a coating on the substrate surface. The resists could be used in the manufacture of electronic parts, for example printed circuit boards. A desired wiring pattern may be imaged in the resist coating on, for example, a copper board. The board is then processed in a developer to remove either imaged or non-imaged portions of the resist coating, and an acid etch then removes only the portions of the copper board no longer bearing the coating.
Thermal lithographic plates are plates which are imaged by infra-red radiation and/or heat. With certain thermal plates, the infra-red radiation may initiate a photochemical reaction in, for example, an onium compound, present in a coating on the plates. In such embodiments an infra-red dye, also present in the coating, acts as a photosensitizer, absorbing the infra-red radiation and sensitizing the decomposition of the onium compound.
So-called CTP (xe2x80x9cComputer-To-Platexe2x80x9d) technology has in part been responsible for the further development of thermal lithographic plates. The required pattern in the coating on the lithographic plate may be xe2x80x9cwrittenxe2x80x9d by an infra-red laser, under digital control.
An exemplary class of thermal plates works as follows:
Plates are exposed to imaging infra-red radiation as described for the first class of plates above. They are then subjected to an overall heating step, after imaging but before development. This is typically referenced as a xe2x80x9cpreheat step.xe2x80x9d This heating step is believed to selectively crosslink those regions of the coating which were selectively imaged, and render them preferentially less soluble in a developer. Thus, on development of such a plate, the regions which were not imaged are selectively developed away. These are negative working thermal plates, and are exemplified by the THERMAL PRINTING PLATE/830 manufactured by Kodak Polychrome Graphics. Such technology is described in U.S. Pat. Nos. 5,340,699, 5,372,907 and 5,491,046. The contents of these patents are incorporated herein by reference.
The most common way of preparing such a thermal plate for a negative working process (so requiring preheating) is to place the plates in very large ovens, which are a separate apparatus from the plate setter and from the processor, thus requiring a large floor space and also high power consumption. Large ovens were desirable in the prior art, inter alia, to passively ensure even heat distribution throughout the plate, by providing a large central region having uniform temperature and ensuring a long dwell time for the plate in the central region. If this does not occur, substantial temperature differences from one part of the plate to another will cause different degrees of crosslinking which can manifest itself as different dot sizes along the plate; but in such printing plates even a lesser temperature difference can be very detrimental. The printing plate may look the same in terms of size of dot after development, but may give poor run length on printing.
Processors are known in which there is an integral preheat section upstream of the chemical development section, but these have been designed for use with conventional negative working plates. xe2x80x9cConventional negative working platesxe2x80x9d refers herein to negative working plates imaged by ultra-violet or visible radiation. With such plates the photo-sensitive coating on the plate is given a top coat of an oxygen migration barrier material, for example polyvinyl alcohol. This must be removed after the preheat and before development, thus the processor must have a wash section between the preheat oven and the chemical development section. The use of a preheat oven upstream of a development section in a single processor is much more straightforward to accomplish with conventional negative working plates than with thermal plates. This is because conventional negative working plates are very xe2x80x9crobustxe2x80x9d in terms of the amount of heat which can be delivered to them before development, and yet are even quality. In other words, there is a very wide operating window in terms of heat input and there is less difficulty in terms of process control, in employing a very simple preheat oven. Despite their robustness, the processing of and the resulting image on such plates can be improved by controlled preheat and baking. With thermal plates the situation is different; as described herein it has been found that the operating window is narrower (in terms of heat input), such that the use of a small footprint preheat oven integrated with a development processor of a type employed to previously in an apparatus intended for development of conventional negative working plates may not be suitable for thermal plates.
Most prior processors do not employ any preheat oven. However products which are believed to employ a preheat oven for conventional plate processing are the PHW 32 processor sold by Technigraph of Thetford, UK and the INTERPLATER 85 HD/135 HD Polymer processor available from Glunz and Jensen.
A particular problem with attempts to develop thermal lithographic plates in processors such as those mentioned above is that the trailing edge of the plate, as the plate proceeds through the processor, becomes relatively hotter than the other portions of the plate and the front edge of the plate may be heated less than the other portions of the plate. This is believed to be because, when using a constant temperature preheat oven heat conduction from heated regions of the plate to regions yet to be heated, which thus enter the preheat oven at a raised temperature, causes a temperature gradient within the plate. In the middle of the plate this does not appear to be as significant a problem because an equilibrium temperature may be reached as heat is conducted toward trailing regions which can act as a heat sink. However at the trailing edge of the plate the heat thus conducted to it can be conducted no further back, and the trailing edge of the plate may become so hot that xe2x80x9cfoggingxe2x80x9d occurs; in effect, the trailing edge of the plate has been exposed to excessive heat causing the coating to crosslink in unexposed areas reducing the differentiation between imaged and non-imaged areas. Furthermore, problems can arise with variations in properties across the plate, due to temperature differences in the transverse direction.
Thus, in using such an integrated preheat/processor apparatus designed for conventionally imaged negative working lithographic plates in attempts to develop thermal at plates, there is a temperature gradient rising, for example from the front to the rear of the plates in the preheat oven and, it is believed, rising exponentially in the rearwards direction. Evidently the conventionally imaged negative working lithographic plates are sufficiently xe2x80x9crobustxe2x80x9d that they can be used in such a preheat oven, but thermal plates are not; the xe2x80x9cfoggingxe2x80x9d at the trailing edge region and lower temperature at the leading edge is often unacceptable.
With many lithographic plates or other resists it may be desirable to provide heating after development, simply to strengthen the residual coating and extend the lifespan of the productxe2x80x94in the case of a printing plate, resistance to aggressive press chemicals, and also the run length. It is the industry norm to employ, for this purpose, post-development ovens which are separate from the processor. These ovens are typically very large and expensive pieces of equipment. The post-development heating step is often referred to as xe2x80x9cbaking.xe2x80x9d
The PULSAR processor offered in the past by Horsell Graphic Industries of Morley, Leeds employed a plate baking oven downstream of the development section. The PULSAR processor was intended to effect post-development baking of conventional positive plates employing quinone diazide compounds and imaged by UV radiation, not thermal lithographic plates.
A significant problem is the amount of space (the so-called xe2x80x9cfootprintxe2x80x9d) taken up by a separate preheat oven, processor and plate baking oven. The xe2x80x9cfootprintxe2x80x9d of these separate items, and especially of the ovens, is large.
It is an object of the present invention to provide an integrated apparatus for the development of lithographic plates, suitable for development of certain thermal lithographic plates at least, and having at least one integral oven.
It is a further object of the present invention to provide an oven with improved performance.
In accordance with one embodiment of the present invention there is provided an integrated apparatus for processing a resist pattern imaged on a substrate, the apparatus comprising a preheat oven having an entrance and an exit which define a pathway along which the imaged substrate progresses, and a chemical development section which is downstream of the preheat oven and which effects the chemical development of the imaged substrate, wherein the preheat oven is capable of actively controlling the amount of heat applied to the imaged substrate as the substrate progresses through the preheat oven so as to heat the imaged substrate evenly.
xe2x80x9cActively controllingxe2x80x9d as used herein means that the amount and/or distribution of heat delivered to the imaged substrate can be varied while the imaged substrate is passing through the oven.
The active control of heat will permit the even heating of the imaged substrate without the need for the large footprint preheat ovens of the prior art.
Preferably the preheat oven is capable of reducing the amount of heat applied to the imaged substrate as the substrate progresses through the preheat oven. It is desirable to reduce the amount of heat applied to the imaged substrate as the substrate progresses through the preheat oven because without this the trailing end region of the substrate will become relatively hotter. As explained above this can cause xe2x80x9cfoggingxe2x80x9d of the class of thermal printing plates which require a preheat step.
Preferably the temperature control within the preheat oven is such that the heat applied to an imaged substrate as it progresses through the preheat oven is reduced, to compensate, at least in part, for the propensity for heat to build up towards the trailing edge of the substrate. Preferably the compensation is such that the temperature along the substrate is substantially constant, or at least is such that there is a reduced temperature variation across the substrate, within limits acceptable for the substrate in question. A temperature difference between regions of the substrate of no more than 30xc2x0 C. may be acceptable. However it is preferably no more than 20xc2x0 C., more preferably no more than 10xc2x0 C., even more preferably no more than 5xc2x0 C., and most preferably less than 2xc2x0 C.
In a preferred embodiment a controllable mechanical barrier is provided or is able to be introduced between heat-emitting parts of the preheat oven and an imaged substrate progressing through the preheat oven, the mechanical barrier being selectively operable or adjustable to reduce the amount of heat applied to the imaged substrate as it progresses through the preheat oven.
Preferably the mechanical barrier comprises an array of louvers able to be turned between positions in which they are arranged generally transversely to an imaged substrate progressing through the preheat oven, such that heat may pass between them, and capable of oblique positions in which they together reduce the delivery of heat to the imaged substrate, and positions parallel to the imaged substrate. In the parallel positions the louvers may together constitute a wall or shutter. The louver edges may contact or overlie each other so that the louvers may act as an effective barrier, at least to radiant heat.
Preferably the mechanical barrier, when employed, is of a material, or materials, resistant to degradation at the temperatures employed. These temperatures are typically relatively moderate, typically 80-250xc2x0 C., for example 120-200xc2x0 C. Another preferred characteristic is that the mechanical barrier (e.g., the louvers) has suitable mechanical properties to work reliably over an extended period. The mechanical barrier may be constituted of a metallic material, an engineering plastics material, a ceramic material, or a reinforced resin, for example a carbon fibre resin, and the like. Preferably, the mechanical barrier (e.g. the louvers) is metallic, and a particularly preferred material having a good strength to weight ratio is titanium.
Although described herein is the use of a controllable mechanical barrier to control the amount of heat applied to the imaged substrate as it progresses through the preheat oven, there are many other ways which will be apparent to those skilled in the art in which the amount of heat applied may be controlled. For example, the substrate pathway may be such that, as it advances, the substrate moves further away from a single matrix heater which is provided. Alternatively, a heater matrix may be itself movable, so that the spacing between a heater matrix and the substrate may be increased. Alternatively the heater matrix may be controlled to deliver a lesser amount of heat. For example a highly controllable halogen heater matrix, of a type similar to that used on cooking hobs, may be employed. The heat may be provided by a hot air flow, and the heater elements which heat up air flowing past them may be operated at lower current; or the fan which drives the air past those heater elements could be turned down. Alternative mechanical barriers to louvers may be envisaged, for example pantograph or bellows-type mechanical barriers, for example extending from one side of the preheat oven across it, or with two parts extending from respective sides, and meeting in the middle. Such embodiments may be regarded collectively as xe2x80x9cmetal curtainsxe2x80x9d (though other heat resistant materials could, in principle, be used). Another possible control method is adjustment of the speed at which the plate progresses through the preheat oven. In particular, it may be desirable to use a slow speed when the leading edge region of a plate enters the preheat oven, then to increase the speed. It may be desirable to employ the fastest speed when or approximately when the trailing edge region of the plate enters the preheat oven. Yet another way in which the heat may be controlled is by employing cooling air, arranged to flow into the region between the plate and the heater matrix, for example in a direction transverse to the direction in which heat flows from the heater matrix to the plate. One or more such control means may be employed.
Preferably the apparatus comprises control means effective to control the amount of heat applied to the imaged substrate automatically in dependence upon the operating speed of the apparatus or parameters of an imaged substrate which is to be developed, or combinations thereof. Such parameters will include the length of the imaged substrate and, in most cases, its thickness, width, or thermal conductivity, or combinations thereof.
The control means may comprise a sensor to sense the passing of the leading edge and/or the trailing edge, or both, of an imaged substrate which is to be developed, and means to increase, reduce or terminate the delivery of heat to the substrate an interval after the sensor has detected the leading or trailing edge (or both) of the substrate. The duration of that interval is preferably dependent at least upon the speed at which the substrate is being conveyed and the length of the substrate. Preferably the sensor is upstream of the preheat oven, most preferably immediately upstream of the entrance to the preheat oven, but could be within the preheat oven or downstream of the preheat oven. Preferably the alteration in heat delivery to the substrate is triggered by passage past the sensor of the trailing edge of the substrate.
Preferably the preheat oven comprises a heater matrix arranged to deliver heat generally perpendicularly to a substrate progressing through the preheat oven. However this is not essential and may not apply if, for example, the heater matrix comprises one or more parabolic mirrors or a hot air outlet or outlets.
The heater matrix may be capable of controlling heat delivery in the transverse direction (cross-wise of the substrate). For example, edge heater tiles or blocks may be arranged not to operate when narrow width substrates are being processed; for other plates longitudinal edge regions may need more heat or less heat than the central region, depending on the circumstances, and this may be controlled by varying the temperature transversely across the heater matrix. Preferably the heater matrix has at least three (3) zones across its width; i.e. two edge zones and one central zone. These zones may thus independently be controlled to yield even temperature conditions across the width of the substrate to facilitate the processing of substrates of different width, or both.
In a preferred embodiment the heater matrix faces the imaged and coated surface of an imaged substrate being conveyed through the preheat oven. Preferably the heater matrix faces generally downwards. Preferably, therefore, the imaged and coated surface of a substrate being conveyed through the preheat oven faces generally upwards.
The substrate pathway through the preheat oven may be defined at least in part by rotatable support means, for example rollers or wheels, which contact a non-imaged surface of the substrate being conveyed through the preheat oven. Preferably the rotatable support means comprises a heat resistant material having a low coefficient of thermal expansion, for example a ceramics material. It will be apparent from the foregoing definitions that this is preferably the downward-facing surface of the substrate.
The heater matrix may comprise a ceramic heater tile or block or an array of ceramic heater tiles or blocks. The heater matrix may comprise any conventional heating mechanism capable of delivering heat to a substrate surface. For example, it could also comprise UV heaters or halogen heaters, carbon element heaters, hot air heaters or any other convenient heat delivery means.
In a preferred embodiment, the preheat section comprises two heater matrices that are arranged above and below the substrate pathway. Preferably, at least one of the matrices is controlled in the manner described above, to reduce the longitudinal temperature gradient or provide transverse control of heating, or both. In another embodiment, both matrices are controlled as described above.
With respect to the present invention, it has been found beneficial, especially in embodiments having heater matrices above and below the plate pathway, for a mechanical barrier to be in a heat-obstructing condition prior to the entry of an imaged substrate, and to move to a less heat-obstructing condition when an imaged substrate enters or is about to enter.
In preferred embodiments of the invention measures are taken to reduce extraneous air flows into and out of the preheat oven. It appears to be deleterious to allow air to flow freely through the entrance and the exit, even though those are typically narrow slots. Preferably, therefore, the entrance, or the exit, and preferably both, are provided with means for hindering or preventing airflow therethrough, preferably such as a resilient body displaced by the oncoming substrate. This may be a textile body such as a fiber curtain or felt but is preferably an elastomeric body, for example a flap. In a particularly effective arrangement, the means for hindering or preventing the flow of air through the opening (that is, the entrance or exit) comprises a pair of displaceable elastomeric flaps each retained along one edge region of the opening respectively above and below the opening, with the other edge region of each flap being free, the free edge regions being adjacent to each other, such that a substrate can pass through the opening by displacing them, with one flap contacting one surface of the substrate and the other flap contacting the other surface of the substrate. An elastomeric flap or flaps may have slits so that a narrow substrate need displace only a section of the flap or flaps between slits, and not the entire flap or flaps.
Preferably the preheat oven has means for cooling the external wall or walls thereof. This means for cooling preferably avoids injection of cooling air into, or withdrawal of hot air from, the preheat oven. Suitably the preheat oven has ducting around it, preferably over its top wall, or its side walls, or both. Preferably the ducting comprises inter-communicating ducts over the top and sides, wherein at least one opening is present in the duct over the top of the of the preheat oven and at the base of the duct on each side, at least one such opening being for inward flow of air and at least one such opening being for outward flow of air. Preferably at least one fan is provided to impel air along the ducting. Preferably at least one further fan is provided, to provide increased air flow in the ducting when an overly high temperature is sensed. The ducting may serve as an insulating jacket around the preheat oven. If desired, the ducting may be used to add heat to the preheat oven, by conveying hot air through the ducting.
The preheat oven preferably has a small footprint. Preferably the pathway length (i.e. the distance between the entrance and the exit) is less than the width of the cavity of the oven. Preferably the pathway length is less than or equal to the length of an imaged substrate, in use (that is, its dimension parallel to the pathway). Preferably, the pathway length is less than 1 m, more preferably less than 0.6 m. Preferably the width of the cavity of the oven is less than 1.5 m. Preferably, to accommodate imaged substrates which are lithographic printing plates the width of the cavity is at least 0.4 m.
For use with some thermally imaged substrates, the substrate pathway between the preheat oven and the development section does not pass through a wash section; this is not typically necessary with thermal lithographic printing plates, in relation to which the present invention is of highest (but not exclusive) interest.
Preferably, the substrate pathway between the preheat oven and the development section passes through a cooling section. The cooling section preferably comprises means for delivering coolant to an imaged substrate being conveyed from the preheat oven to the development section. The coolant may conveniently be air, and it is normally adequate if the coolant, preferably air, is at ambient temperature. The coolant may be a series of fluid jets directed onto the substrate or, preferably, may be a sheet of coolant directed onto the substrate, for example issuing from a slit arranged transverse to the plate pathway. Thus, a preferred cooling device is typically termed an xe2x80x9cair knifexe2x80x9d directed onto the surface of the substrate.
Preferably, means is provided for shielding, or thermally isolating, the exit of the preheat oven from the coolant. This is to reduce the possibility of the cool air entering the preheat oven through the exit, and the alternative possibility of withdrawal of hot air from the preheat oven by the Bemouilli effect, arising from the flow of the coolant air past the exit. This may be accomplished by a flexible barrier, for example a brush or other textile body. Most preferred, however, is an elastomeric flap which extends between the coolant outlet and the plane of travel of an imaged substrate in the region of the means for delivering the coolant, whereby the flap contacts an imaged substrate moving through that region, and the substrate and flap together prevent coolant from reaching the exit of the preheat oven.
The processor of the invention may include a baking oven downstream of the chemical development section. The baking oven may have the control features of the preheat oven described above and the definitions given above may be applied to the baking oven. However the baking oven may be a simple oven without such controls, because baking of items such as printing plates to improve on-press life is typically a robust process, not requiring careful control.
In use, the integrated apparatus has the preheat oven adjacent to the chemical development section. The apparatus of the invention may be built as one unitary part or may be built in two parts which the user puts together in abutting relation. The user may thus separate the parts and use them independently, if desired.
In accordance with a second embodiment of the invention, there is provided an integrated apparatus for processing imaged substrates, the apparatus comprising a preheat oven, a chemical development of the imaged substrate, and a baking oven downstream of the development section. Preferred features of the apparatus of this second embodiment of the invention are as defined above, and in the claims, in relation to the first embodiment of the invention.
In accordance with a third embodiment of the invention, there is provided a method of developing a resist pattern on a substrate, preferably a thermal lithographic printing plate, which has been imaged, the method comprising conveying the plate through the apparatus as defined or described herein in relation to the first or second embodiments of the invention.
In accordance with a fourth embodiment of the invention, there is provided a method of preparing a lithographic printing plate for its end use, the plate having an imagable coating on substantially the whole of a surface of the plate, the method comprising imaging the plate by exposing the surface to an energy pattern and developing the plate in an integrated apparatus defined or described herein in relation to the first or second embodiments of the invention.
In accordance with a fifth embodiment, there is provided an integrated apparatus for processing a resist pattern imaged in a substrate, the apparatus comprising a preheat oven having an entrance and an exit which define a pathway along which the imaged substrate progresses, and a chemical development section which is downstream of the preheat oven and which provides the chemical development of the imaged substrate, wherein the preheat oven has surrounding or adjacent ducting, and means to convey air through the ducting, to withdraw heat from the oven or add heat to the oven.
In accordance with a sixth embodiment there is provided an integrated apparatus for processing a resist pattern imaged in a substrate, the apparatus comprising a preheat oven having an entrance and an exit which define a pathway along which the imaged substrate progresses, and a chemical development section which is downstream of the preheat oven and which provides the chemical development of the imaged substrate, wherein the entrance to the preheat oven comprises a barrier for hindering or preventing the flow of air through the entrance, and the exit from the preheat oven comprises a barrier for hindering or preventing the flow of air through the exit.
In accordance with a seventh embodiment, there is provided an oven having an entrance and an exit which define a pathway along which a body to be heated may be progressed, wherein the oven is capable of actively controlling the amount of heat applied to the body as it progresses through the oven so as to heat the body evenly. The preferred features described above in relation to the preheat oven of the processor of the first embodiment of the invention apply to this embodiment of the invention.
In accordance with an eighth embodiment of the invention there is provided an oven having an entrance and an exit which define a pathway along which a body to be heated may be progressed, wherein the oven has ducting around it, and means to convey air through the ducting, to withdraw heat from the oven or add heat to the oven.
In accordance with a ninth embodiment of the invention there is provided an oven having an entrance and an exit which define a pathway along which a body to be heated may be progressed, wherein the entrance to the oven comprises a barrier for hindering or preventing the flow of air through the entrance, and the exit from the oven comprises a barrier for hindering or preventing the flow of air through the exit.
In accordance with a tenth embodiment of the invention there is provided a substrate produced by any of the methods or using an integrated apparatus of the invention, as defined herein or described herein.